You can imagine the sign hanging outside Tory Central Office: 'Shop closed - out to lunch'

Lord Heseltine

Also joining the wrangling has been former chancellor Norman Lamont who attacked John Major, a supporter of Ken Clarke, for his "regrettable" trashing of Lady Thatcher and her legacy.

Europe remained the dominant theme - as it was in Wednesday night's live studio debate - despite both candidates calling for Tories to move on from the subject.

Writing in London's Evening Standard, former deputy prime minister Lord Heseltine urged party members not to vote for Mr Duncan Smith and "disqualify ourselves from power for the next 15 years or more".

Hague's 'Europhobia'

"You can imagine the sign hanging outside Tory Central Office: 'Shop closed - out to lunch'," he said.

And he attacked outgoing leader William Hague - who has endorsed Mr Duncan Smith - for ditching long-standing Conservative pragmatism on Europe.

"It was replaced by a brand of Euroscepticism, indeed Europhobia, unknown in this country since I first stood as a Conservative candidate in 1959," Lord Heseltine said.

"The result was a disaster - 2001 produced the worst Tory result in living memory."

William Hague: Europe policy attacked by Lord Heseltine

Mr Clarke returned to the European theme on Thursday too, telling BBC Radio Cornwall during a campaign tour of the South West.

He said: "The idea we should go further to the right and become even more Eurosceptic is, I think, a suggestion that will put us in opposition for a generation."

During Wednesday's head-to-head clash Mr Clarke accused his rival of being one of the most "Europhobic" MPs in the Conservative Party.

But Mr Duncan Smith, who said he would "never" vote in favour of the euro, warned that - if elected leader - Mr Clarke's opposition to the majority view of his party would be exploited by Labour "and we will break up over this for four years".

"The only way we can settle this is by leading through the majority of the party who don't want to join the euro," he said.

Unity pledge

Despite the heated exchanges on Europe, overall the debate was considered to be gentlemanly - both ended by stressing the need for unity and pledging to fight together against Labour.

Lord Lamont said it was "remarkably friendly and remarkably constructive" and denied the increasingly bitter feuding surrounding the contest would damage the Tories.

However, he engaged in some sniping of his own when asked about Mr Major's comment that Lady Thatcher had caused "immense damage" to the administration in which both men served.

An election is about fighting to get a result and I think when it is all over the party will come together

Lord Lamont

"John Major's difficulties as prime minister weren't caused by Mrs Thatcher, they were inherent in the situation," Lord Lamont told BBC Radio 4's Today programme.

"I don't think he did any good, I don't think he did himself any good, by resurrecting this."

The continuing debate at the party's highest level comes as many of its 300,000 grassroots members are expected to start voting.

Party vice chairman Steve Norris said they expected the "vast majority" of ballots returned within days, despite the deadline being more than two weeks away.

At the same time he dismissed as isolated examples evidence that ex-Tories have been sent voting forms while some fully paid-up members had received none.

"The reality is I don't think any of this is going to have the slightest impact on the outcome," he said.